1. Field of the Invention
The present invention relates to supported anisotropic reverse osmosis membranes based on synthetic polyamides and processes for their preparation.
2. The Prior Art
It is known that many synthetic polyamides may be used with success in the preparation of anisotropic membranes for reverse osmosis, which membranes have good salt rejection characteristics and a high water flux.
Unfortunately, such membranes do not possess altogether satisfactory mechanical properties. In general, serious difficulties are encountered in assembling these membranes into particular types of reverse osmosis modules such as tubular, spiral and the like modules.
In order to overcome these disadvantages, numerous procedures have been suggested. One such procedure involves the use of supported polyamide membranes, namely membranes containing homogeneously incorporated therein a support such as cloth, fabrics or fibers of various types, shapes and sizes, for improving the physical-mechanical characteristics of the membranes and for facilitating their assembly into reverse osmosis modules of any type, shape and dimension.
This procedure, however, is not altogether free of disadvantages. In fact, in the case of processes for the preparation of membranes in which the solution is spread on one face of the support or carrier, or the support is immersed or dipped into the polyamide solution, it has been observed that a thick layer of polymeric material, which will seriously compromise the subsequent working of the membrane, forms on both faces of the carrier. In fact, such a membrane initially exhibits low water flux and high salt rejection and then, due to the accumulation of salt around the membrane, the phenomenon is reversed and it exhibits low saline rejection and high water flux.
In the case of processes consisting of laying the support or carrier on a glass sheet and subsequently spreading over the glass sheet a polyamide solution (which is then followed by evaporation of the solvent and the coagulation in an aqueous medium of the membrane) the polyamide solution penetrates into the meshes of the support at the moment of the spreading, this, of course, causes the same disadvantages as in the previously noted situation. Moreover, part of the air trapped between the glass sheet and the support remains incorporated in the polyamide solution and leads to the emergence of air bubbles which, in turn, produce superficial faults in the membrane during the evaporation of the solvent. This causes a serious deterioration in the flux rate and/or saline rejection characteristics of the membranes.
It is an object of the present invention to provide a process for the preparation of supported anisotropic reverse osmosis membranes based on synthetic polyamides, and the membranes themselves, which are free of the above-mentioned disadvantages.